1. Field of the Invention
The present invention relates to an optical scanning device for scanning a plurality of media and an image forming apparatus using the optical scanning device.
2. Discussion of the Background
A color electrophotographic image forming apparatus includes in a housing a plurality of optical devices which respectively emit as scanning beams a plurality of laser beams output by a plurality of light sources. The scanning beams emitted from the housing scan their corresponding photoconductors, serving as a plurality of media, to form thereupon latent images of an image in their corresponding colors, respectively, each corresponding to a different color. The latent images are developed, i.e., made into visible images, with developers of corresponding colors, respectively, and the developed images are sequentially transferred onto a transfer sheet (recording sheet), superimposing one upon another, thereby forming a color image on the transfer sheet.
More specifically, a color electrophotographic image forming apparatus such as a digital copier and a laser printer includes four photoconductor drums arranged in a direction in which a transfer sheet is conveyed. Charged surfaces of the photoconductor drums are exposed by scanning beams corresponding to yellow, magenta, cyan, and black, respectively, so that latent images corresponding to yellow, magenta, cyan, and black are formed thereupon. Then, the latent images are developed into visible images by developing devices using developers of corresponding colors. The visible images are sequentially transferred onto a transfer sheet, superimposing one upon another, thereby forming a color image on the transfer sheet.
In the above-described color image forming apparatus, for scanning the four photoconductor drums with laser beams, respectively, a plurality of scanning devices, one for each photoconductor drum, are used. Accordingly, a large space is necessary, and thus the size of the image forming apparatus is increased.
Japanese Patent Laid-open publication No. 4-127115 discloses an optical scanning device for color image forming apparatuses in which a plurality of optical beams are incident on a single deflector and respective scanning image forming optical systems including mirrors are arranged in a vertically layered manner so that the whole parts of the optical scanning device are accommodated in one housing.
FIGS. 5 and 6 illustrate an optical scanning device described above. FIG. 5 is a plane view for explaining arrangement of a reflecting device and an optical detecting device in the optical scanning device, and FIG. 6 is a side view illustrating a relation between the optical scanning device and each photoconductor drum.
In the optical scanning device illustrated in FIG. 5, laser beams are emitted from laser light sources 100, and shaped by a coupling lens 200, respectively. After passing an aperture 300, a cylindrical lens 400, serving as a first image forming optical system, forms a respective one of the laser beams into a line image long in the main scanning direction in the vicinity of a deflecting reflective surface of a polygon mirror (500a) of a rotating deflector 500. Then, each of the laser beams is deflected by the polygon mirror (500a) so as to sweep a predetermined plane. Further, first and second scanning lenses 600, 700, serving as a second image forming optical system, and three mirrors 800 (illustrated in FIG. 6) project the laser beam on a surface 1200 of a photoconductor drum 1100, thereby scanning the surface 1200.
In the optical scanning device illustrated in FIG. 5, two laser beams emitted from different laser light sources 100 are incident on different deflecting reflective surfaces of the single polygon mirrors (500a) of the rotating deflector 500 at the same time. Thus, the two laser beams are deflected simultaneously by the single polygon mirror (500a).
Referring to FIG. 6, the rotating deflector 500 includes the polygon mirror (500a) and another polygon mirror (500b) arranged in upper and lower steps, respectively, and two optical systems projecting the laser beams from the two laser light sources 100 on the surfaces 1200, respectively, are provided for the polygon mirror (500a) at the upper step and two other similar optical systems are provided for the polygon mirror (500b) at the lower step. Thus, four optical systems in total are arranged in one housing.
Thus, the above-described scanning device scans the surfaces 1200 of the four photoconductor drums 1100 at the same time with four laser beams emitted from four laser light sources 100 for forming images of magenta (M), cyan (C), yellow (Y), and black (BK), respectively. In FIG. 6, colors of images formed on the four photoconductor drums 1100 are indicated by M, C, Y, and BK, respectively.
In the optical scanning device described above, to detect each timing of scanning the surfaces 1200 with the laser beams, generally, synchronizing sensors 1000 are provided as illustrated in FIG. 5. Further, synchronizing mirrors 900 are arranged in the vicinity of both ends of optical paths of the laser beams scanning the surfaces 1200, respectively, so that the laser beams deflected by the deflector 500 are reflected toward the synchronizing sensors 1000, respectively.
However, providing a synchronizing mirror 900 and a synchronizing sensor 1000 for each of the four laser beams increases the cost of the scanning device. Further, the size of the housing for the scanning device is increased, leading to increase in the size of an image forming apparatus using such a scanning device.